How can betrayed administrator learn to trust again?

Q: I’m an assistant principal in a high school, and I’m in the applicant pool to become a principal. I’ve always had a strong, trusting rapport with a colleague who used to work with me. She was a teacher then, but she’s now an assistant principal at the high school where I hope to be placed. I actually helped her get that job — I gave her a glowing reference, and I meant every word. We worked well together, and I respected her, valued her opinion, and considered her a friend.  

I know for a fact she has no interest in being a principal, so I thought nothing of asking her to help me get a foot in the door. She enthusiastically agreed and said she’d love to work with me again. She also made a point of saying she considers me “incredible principal material.” So imagine my surprise when I learned from someone senior to both of us that she wasted no time torpedoing my application. My source showed me evidence in writing. When I mentioned this to one of my current colleagues, she was not surprised. She said that this person has undermined me before. I’m angry, bewildered and upset, and I’m also concerned about my obliviousness. 

Not long after I found out the truth, my former colleague made a point of telling me she had supported my application. I didn’t bother confronting her about the lie, because the damage had been done. I know you recommend assuming positive intent, and I always have, but clearly that’s backfired on me here. Going forward, how can I make sure I trust the right people? And how should I handle this particular person? I don’t want anything like this to ever happen again, but I also don’t want to walk around completely paranoid. I’m quick to trust and forgive, and maybe a little naïve — it’s just who I am. This has been a very hurtful wake-up call, however, that I need to fine-tune my “trust radar.” Or at least put some safeguards in place. Your thoughts?